Hi
Bear in mind that I'm not one of the vastly experienced guys on this group, but here's a few things to try. Obviously these are things that I've screwed up recently enough to remember...
1. When you dye something, wipe off the excess, that leaves really nice big swirly streaks.
2. Did you use the de glazer stuff? I haven't tried that yet. It seems to me that having a part of the leather that is somehow different (coated, thicker, more dense ...) it will seem a different color which would look good if it was lighter brown,but look bad in black.
3. Putting olive oil on it won't help.
4. Did you try wetting it before you put the dye on?
That's my $0.02 and worth half that.
Later